This invention relates in general to electrical shielding, and more particularly to a shielded structural or containment member, a member of that type used as part of an aircraft, and a process for producing such a member.
Lightweight nonmetallic materials, such as graphite epoxy composites, are finding widespread use in aircraft because of their high strength-to-weight ratios and their ability to be formed into complex configurations without expensive machining operations. However, these materials in and of themselves are quite vulnerable to lightning because, being generally nonconductive, they cannot dissipate electrical charges. Indeed, lightning will normally puncture and severely weaken a graphite epoxy composite. A puncture in the fuselage of an aircraft could cause the cabin to lose pressure, or much worse it could weaken the fuselage to the extent that it cannot perform its structural function. Also, a puncture near an electrical component may cause that component to malfunction.
One procedure for protecting a composite member involves adhesively bonding a thin foil or screen of aluminum to the outwardly presented surface of the member. Thus, in the event of a lightning strike, the electrical charge will dissipate through the metal of the foil or screen and will not concentrate at one specific area as would be the case without the metal shield. Sheet titanium may also be used for the shield. However, bonding a foil or screen to a composite is not easily done, and furthermore, the bonding is limited to flat or gently contoured parts, and certainly cannot be employed with parts having intricate contours. Also, foil bonding techniques are costly. Furthermore, the adhesive adds significantly to the weight of the shielded composite member. Aside from the foregoing problems, aluminum and carbon, the latter being the principal component of graphite epoxy composites, are not compatible from a chemical standpoint, and as a consequence the aluminum corrodes quite rapidly in the presence of the carbon. Titanium, on the other hand, is extremely expensive and difficult to work.
Another procedure currently used to protect composite members is to apply a thin coating of aluminum to the outwardly presented surface of the composite by arc, flame, or plasma spraying. Each of these techniques requires bringing the aluminum to a molten condition, and then transferring it to the surface of the composite. Aluminum melts at about 1200.degree. F. and the transfer of the molten aluminum to the surface of the composite can cause thermal damage, particularly in the case of composite panels.
To avoid thermal damage, aluminum is sometimes applied to the mold in which the resin of the composite is cured, in which case the aluminum will bond to the resin. This process, which is known as transfer molding, is considerably more expensive than spraying the aluminum directly onto the cured composite material.
Because of its high melting temperature, titanium cannot be applied to the composite material by metal spraying techniques, but instead must be applied in sheet form.